Method of operating electrical space discharge devices



J Apnl 20, 1948. L. K. MARSHALL 2,439,816 METHOD OF OPERATING ELECTRICALsmcm msbmmn DEVICES Filed Sept. 10, 1942 PULS E TE! 695250 /6 DELAY/N6Pom/1? DEV/CE l I SUPPLY /8 v I L r 0 r OSCILLATOR 5- Pl/LSl/VG DEV/CE IVACUUM UBE REL/l Y INvENToz. LAURENCE K. MARSHALL,

Patented Apr. 20, 1948 nee K. Marshal], CambridgerMess assignmi r rLaure Appligation Septe This inr m n rela es o. m hqd 92%? 3% rel rtr lspa e d smr e (Rimes, 9i th We ha g d rqi at d hlefi i ni etheee ee asmemory oathodes.

E9: rta i rp e 149 si r exem lem PH L pp ica s ubes a e re ir 1. intermew c ndu pee urr n s TQ;S?1?.I. Y= 497b peak ur r q irem nts t. as, erebfior been the igrac tice' to use tubes of relatively large cure-wndi tfi capa ty bq wh e cam-4 ode at their usual operating temperature supplyq i zfi em s n 9? $1 9 ee s urren r uireo i e. qcqr e o e Pract ce. uchathpde e iel'q es afi a their 4 3. 2 opera e ereb kled ie 99 peninqfiedrvnpn-leeek De: p qe H wev r he v cr is ef c ent b Qws during nn-Peek fl s. he, tem ra ur 'iw' w ee/ mission excefiq th men-peekrequjremefite. Furthermore, in relep ion to, n on pik eriod requirementsthe tu beg employed. in

An obj qt. of the present inyentiofi'i'sftfo. proviqejegi n re.efficient method of 'oijeret'in tubes, peyrticu ev y n sfiem's where ides are I' quird to conduct peak currents whoe 'duraltion 1. mencompared; with theduraltion of henom p n v her object ofi thepresent i rl vention is the prov: o' n oi a method ofoperatin tubes irifslqph. mieh' E ble the- 45 f sma erfl lii tih heretofore'requird. f0 nary at tmptg to use cathodes, havin high "emisgip'r f currents intro'cme thealgae; i e emmfiion i a lhode a e al. "A sj il ljf ulffihlf bject;'thereimie i tofjoperaite a itien. caighodeeb high peak emission currenwith sgbstanpiel' reduc on n' he va oration. ofcethode .rr

7 I further objects of theprQsent i1 ve ion 1- become app rent a d' 'h iwill be best understood from tl ii'fQllpw m nufacturin'g Cohili' y,

, emen mber 10, 19112, Serial No. 457,803. 2 C gigl s. (Cl. 250 27) wany r pt on o n mmpl fise qn. e fi reference in l 9 h d wine i hi hf h ls he ifiefiiq d ee emu s st m fi iz p my ent nle t' I-QQiQ tub s a desin d ha h i th d operated t a 5 6 at df empe e- Lure. A1, the ratedtempera-gum a give'njr'ete, of emis qnf s r0s1ue- I h c t q e; is oprate a e 1; ra d trempeltewr z n inpr e e n e I wi oec r- This. ire ex res d whe fi e t d i cha q qeequatiq a mews- Is=AT ein whiph S is thesetfiration current per uni}; area, of emitter, T is the absolutetempereture, w

11s the electronic minim or the, emifiterKwork' funefiibml, leie'. Boltnnsr universal gas Qn sm t; K15; a onstan'n r I th emitting magma, d''is the'Nabierian bkis ef' It Will be apgirei lt.

are r. this equation that a slight r ar 1' i 'rbduc' i a large inp ease.nit-he rate ofrein i si fi "However, when atb dde f F m e their rated eture elrfiai n dleterio'us'jefieo ensue, such aslfor exvegnplegfeveliorition ofthe apnoea material arid,

e? a n were f .pi f h v ii coated be Iipdes is .1n dilnately sfiortefieddue to me rapid wi'alportion or 'the coating m lter ial at such highertemperatures. The ratedtempera tuiife of @c'athodeis theretore usually?camrqmise"r e ;nfi fg' the maximum emissibg obtaiinfaple underbrdinalrj} methods of operapi'on withouttheihfirodyctiofi of substantialdeleterious effects; 'zmd'wim ellowanee for a reas na long operativelife, such as, for exfe imple, 1000 hours 6r severl tihoilsandhours.

e el fr h rea r em ss n bt inab e a temperatures, above the ratedtemperature has beer; considered practically unavailable beta-135s oftheaforementioned deleterious efiect Howve'r, TI have fl iscqvered how toobtain th is greater. ilis' pnhi ev s h t np a pidine i hes effebt'5 Myinvention wiube' b st. understoodjin, onn ion. Withihe acom a y ng' d l'Hg n which the systemihere illustrated includes an osg ancir' 'edinonjun tion with w lsi de e'rz Th 'puls n device; mayb 91 an wel' knowr;typev such isused for any sgitab le purmentary cathode. Tube 3 isprovided with an anode 5 and may be provided with a suitable controlmeans, such as a control grid 6.

Since this invention relates particularly to the method of operating thecathode of such tubes, the details of the pulsing device and theoscillator are here omitted, especially since such details will bereadily apparent to those versed in the art. This description istherefore directed to the method of operating the cathode.

The cathode 4 may be supplied with a steady current, alternating ordirect, to heat it to a temperature below that required for peak-currentrequirements. This may be its rated temperature or below its ratedtemperature. At this temperature the cathode only produces sufiicientemission for non-peak current requirements. For this purpose anysuitable source of current may be utilized, preferably a direct current,which may be derived from a battery 1. In order to heat the cathode 4 toproduce the peak emission required, I prefer to utilize the dischargefrom a condenser 8. The capacity of condenser 8 is determined by theduration of.the peak pulses to be conducted by tube 3, and the energy tobe supplied to the cathode. In the embodiment illustrated, condenser 8may have a capacity of the order of /2 a microfarad and be charged to avoltage of the order of 1000 volts. A charging voltage of about 2000volts could be conveniently utilized. The temperature of the cathode issubstantially raised by the energy of the discharge, and thisrise intemperature may be of the order of several hundred degrees centigrade,the heat expended being approximately .3 joule. These constants, ofcourse, must be selected in accordance with the specific conditionsencountered.

It is preferred that condenser 8 supply energy to produce the requiredpeak emission only during the time that such peak emission is required.Since the time constant of a condenser discharge circuit is equal toRXC, it will be seen that the resistance of the particular circuit intowhich the condenser discharges, as well as the capacity thereof,determine the rate of discharge. By choosing the proper constants forthe condenser and the circuit into which it discharges, it will bereadily apparent that the discharge of the condenser may be so timed asto produce the desired peak emission only during such periods as suchpeak emission is required.

In the illustrated pulsing system, each peak pulse may have the durationof the order of a microsecond, there being approximately 1000 suchpulses per second, each pulse consisting of oscillations having a highfrequency, such as, for example, from 300 to 1500 megacycles. Theeffective duration of each condenser discharge may therefore be of theorder of a microsecond. The time intervening between shots of energysupplied to the cathode will be of the order of 1000 microseconds, andthe duration of such shots of energy relative to the time between shotswill be approximately of the order of 1 to 1000. It is of courseapparent that these constants may be variedvwithin considerably widelimits depending upon the particular conditions to be met. For example,the ratio between the duration of the shots of energy for heating thecathode and the time between such shots may be as low as 1 to 100, andpractically may be as high as one to several thousands.

While it is desirable for maximum efilciency that the shots of energysupplied to the cathode shall not continue for a period longer than theperiod during which peak emission is required, such shots may have aconsiderably shorter duration than the time required for peak emission.Since a definite amount of time is required for the energy supplied tothe cathodes to be dissipated and for, the cathode temperature to fallto a temperature of non-peak emission, advantage may be taken of thisthermal lag by supplying the energy required for peak emission during aperiod of time considerably shorter than the time for which such peakemission is required. Due to the thermal lag the temperature of thecathode will not drop immediately and the emission of said cathode willbe gradually reduced over a period of time which by proper proportioningof the constants of this system will be suflicient to supply peakemission during the entire period in which such emission is required.

In order that the deleterious effects attendant upon operation of thecathode above its rated temperature be avoided, or kept to a minimum, itis desirable that during periods where peak emission is no longerrequired, the temperature of said cathode should fall rapidly to itsrated temperature, or even below that. For this purpose it is desirableto utilize a tube having a cathode which will rapidly dissipate heat.Such a cathode preferably has the maximum surface for the minimumvolume. One form of cathode satisfactory for this purpose is the thinribbon type of filament.

It is preferred that condenser 8 be discharged through a low impedancepath so that the discharge will have a steep wave front. This steep wavefront takes advantage of the skin effect, concentrating the energyderived from condenser 8 at the surface of the cathode at precisely thepoint where emission occurs. The temperature of the surface of thecathode is thereby practically instantaneously raised to the temperaturerequired to produce the desired peak emission while less of the energyof the discharge is wasted in unnecessarily heating the interior core ofthe cathode.

Since condenser 8 is to be discharged at a rapid rate I prefer to use avacuum tube relay 9 to control the discharge thereof. Since the ordinaryvacuum tube relay may be incapable of handling the required amount ofcurrent necessary to produce peak emission from the cathode 4, I preferto have the condenser 8 discharge into the primary 10 of a step-down aircore transformer I l. Thus, the vaccum tube relay 9 is only required tohandle comparatively small currents at comparatively high voltage, thelarger currents required for peak emission being generated in thesecondary 12 of the step-down transformer H. The secondary 2 may consistof only a few turns, and in order to reduce to a minimum any leakageinductance with its attendant impedances the primary in and thesecondary [2 are as closely coupled as possible. For this purpose thesecondary l2 may consist of one or more turns of a conducting ribbonclosely wound around the primary H1. The secondary l2 of the step-downtransformer II is preferably arranged in series with the cathode 4 andthe battery 1. Because of the high capacity of the battery I, it willoffer little impedance to the pulses of current generated in thesecondary l2. Condenser B may be charged by any suitable source ofdirect current I-4, such as, for example, a battery, rectifier, ordirect current generator, etc. A current limiting resistance 15 may bearranged in series with the source of direct current It.

The operating power as contrasted with the cathode heating power issupplied to the oscillator i from a triggered power supply l6 of anywell-known type. The device I6 is triggered off for the proper length oftime at the proper time intervals by triggering impulses suppliedthrough lines [1.

If, in heating cathode 4 to produce peak emission, a thermal lag isfound to exist which would substantially delay the production of peakemission and thereby affect the operation of the system, this may becorrected by causing condenser 5 to discharge a short'interval of timebefore peak current is required from tube 3. For this purpose a suitablepulse-delaying means 58 may be connected between the pulsing device 2and the lines l7, and may be adjusted so that the vacuum tube relaycauses condenser 8 to discharge a short time before tube 3 is requiredto conduct peak current. Such pulse-delaying devices are, of course,well known, and need not be described in detail herein. A quite commonmeans for delaying the pulses is to employ the equivalent of a longtransmission line made up of a plurality of condensers and reactors. Theoutput of the oscillator I may be delivered to any suitable utilizationdevice, which may be, for ex ample, a radiating system 19.

By the method hereinabove described, it is possible to obtain severaltimes the normal peak output of a tube in such systems. This high outputis obtained without inordinately shortening of the life of the tube andwithout other substantial deleterious efiects. I believe that this isdue in part to the relatively short duration of the pulses of currentsupplied to the cathode and also to other factors, such as, for example,the steep wave front of such pulses. It is to be noted that in theinstances cited, the time intervening between pulses of current suppliedto the cathode is considerably greater than the duration of said pulses.Thus the temperature of the cathode may fall back to approximately itsrated temperature or perhaps below its rated temperature during the timeintervening between successive pulses. The average temperature of thecathode may be below or substantially equal to its rated temperature.

. In the system hereinabove described, pulses of current for heating thecathode are used in conjunction with an additional source of heatingcurrent. With certain types of tubes and in certain systems it isfeasible to use pulses of current without any additional source ofcurrent for heating the cathode. By proper timing of the discharge ofthe condenser, or other means utilized for supplying pulses of currentto the cathode, the cathode may be heated so as to produce the desiredpeak emission only at such times as such peak emission is required.

As has been pointed out before, the rate of emission increases morerapidly than does the temperature. Therefore, it is possible by mymethod to obtain an average emission considerably greater than theaverage emission obtained by operating a cathode at its ratedtemperature. While I have described my method as applied in a system inwhich peak pulses occur intermittently, it is feasible to employ mymethod to obtain a higher output from a given tube in any system wherethe fact that the emission is produced inintermittent peaks will notprevent proper operation of such system and Where such system is able toutilize the increased average emission.

While I have described my method in conjunction with one arrangement forsupplying pulses of current to the cathode, it is obvious that otherarrangements utilizing my method may be employed. For example, I haveshown the source of the pulses of current supplied to the cathode asbeing a condenser. Obviously other electrical energy sources may beemployed, such as, for example, a high frequency pulsator or oscillator.Likewise various forms of cathode and heating means therefor may beutilized. For example, the cathode may be provided with a separateheater to supply the bias heating power while separate current pulsesmight be passed to the cathode itself to produce the high temperaturepeak. Also the cathode might be of the hollow sleeve type heated by highfrequency induction currents supplied thereto in pulses. Numerous othermodifications will readily suggest themselves from the foregoingdescription. It is accordingly desired that my invention be given abroad interpretation commensurate with the scope of the appended claimsand the status of my invention within the art.

What is claimed is:

1. In a system for producing electrical impulses including an electricalspace discharge device having a heated cathode and an anode, means forintermittently applying voltage pulses between the cathode and anode,and means for intermittently supplying pulses of heating current to saidcathode to raise said cathode to a temperature of copious thermionicemission substantially in phase with said voltage pulses.

2. In a system for producing electrical impulses including an electricalspace discharge device having a heated cathode and an anode, means forintermittently applying voltage pulses between the cathode and anode,means for supplying heating current to the cathode to raise said cathodeto a temperature at which the emission of said cathode is insufficientto supply the required peak currents during said voltage pulses, andmeans for intermittently supplying pulses of heating current to saidcathode to raise said cathode to a temperature of copious thermionicemission substantially in phase with said voltage pulses.

LAURENCE K. MARSHALL.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,303,184 Ehret May 6, 19191,533,157 Beatty Apr. 14, 1925 1,765,887 Scott et al June 24, 19301,797,976 Fitzgerald Mar. 24, 1931 2,275,941 Bostwick Mar. 10, 19421,497,948 Shoenberg June 17, 1924 2,275,581 Barton Mar. 10, 19421,881,645 Jones et a1 Oct. 11, 1932 1,903,420 Badma Apr. 11, 19332,052,725 Van B. Roberts Sept. 1, 1936 2,373,543 Cooper Apr. 10, 1945

